Universal Mobile Telecommunications System (UMTS) is one of the third generation (3G) mobile telecommunications technologies and is also being developed into a 4G technology. UMTS terrestrial radio access network (UTRAN) is a collective term for a core network including base stations, known as Node Bs, and base station controllers, known as radio network controllers (RNCs), which make up a UMTS radio access network. A UTRAN can carry many traffic types, from real-time circuit-switched traffic to IP-based packet-switched traffic. A UTRAN allows connectivity between user equipment (UE) and the core network. The RNC provides control functionalities for one or more Node Bs. A Node B and an RNC may be combined in a single device, although typical implementations have a separate RNC located in a central office serving multiple Node Bs.
Recently the evolution of mobile access points within the mobile telecommunication industry has introduced femtocells into wireless communication systems. A femtocell is a small cellular base station, typically designed for use in a home or small business. Femtocells generally connect to the service provider's network via a customer's broadband connection, such as a Digital Subscriber Line (DSL) or cable broadband connection. A femtocell allows service providers to extend service coverage indoors, especially where access would otherwise be limited or unavailable. In 3G terms, femtocells are called Home Node Bs (HNBs).
HNBs are often arranged in uncoordinated large-scale deployments of several HNBs, and therefore the connection to the operator's core network needs to be realized efficiently. A closed subscriber group (CSG) is a specific group of UEs permitted access to a HNB. A CSG identifier (CSG-ID) is broadcast from the HNB in a system information block message (SIB), and only those UEs that are members of this group, as defined by a CSG whitelist of CSG IDs (generally stored on the UE), will attempt to select the cell.
Before deciding to hand over a UE to a HNB, a UTRAN generally needs to acquire UE measurement information related to the target HNB cell. The UTRAN can control what measurements the UE performs. In order to allow the UE to make those measurements efficiently, proximity detection can be configured within the UE via a radio resource control (RRC) measurement control message issued by the UTRAN. One type of measurement sent to a UE contains “CSG proximity detection” information, which is used by the UE to enable a proximity detection function to enter or leave one or more HNB cells within the UE's CSG whitelist. HNBs detected by the UE can be reported by the UE to the UTRAN via a proximity report in a measurement report message.
The 3rd Generation Partnership Project (3GPP) specifications define four RRC states in the connected mode: CELL_DCH, CELL_FACH, CELL_PCH, and URA_PCH. These states reflect the level of UE connection and the transport channels that can be used by the UE. For example, the CELL_FACH state, CELL_PCH state and URA_PCH state are characterized by the fact that there is no dedicated transport channel. In contrast, a UE in the CELL_DCH state has an assigned dedicated transport channel. A dedicated transport channel is not allocated in CELL_FACH, where a default common or shared transport channel is assigned. The descriptions for RRC layers are detailed in specification “3GPP TS 25.331 Radio Resource Control (RRC)” and hence are not repeated in detail.